Response variability is a fact of the brain. There can be dramatic differences in the responses of any given neuron, to any given stimulus, at different moments in time. Different neurons in any cortical area contributing to the representation of any given stimulus or action commonly have substantially different responses. Variations in distributed local, system, and brain-wide responses representing any given stimulus in any given behavioral context can differ radically in different individuals. At the same time, the brain operates with the maintenance of perceptual constancy, cognitive reliability, and learned-behavior stereotypy. How do we account for the robust behavioral representations of inputs and actions in the face of the marked response variability of their neurological representations? This project will address 3 issues. First, it will determine the basic consequences, for neurological response variability, of exposing neonatal rats across the critical period with stereotyped vs naturally variable complex acoustic (speech-like) stimulus sets. Second, it will determine whether or not and how systematically varying the modulatory inputs enabling learning-induced plasticity in adult brains contribute to distributed neuronal response variability and coordination, and to behavioral response variability, in an auditory stimulus recognition task. Third, it will investigate the relationships between variation in neuronal responses in the primary auditory cortex (A-1) and in "secondary" auditory cortical fields (PVAF;AAF;PAF;PPVAF), as a function of stimulus repertoire complexity, in trained adult rats. The long-term goal of this project is to determine how a neurological strategy of learning-driven abstraction and coordination can lead to new insights into how we can potentially revise learning strategies to improve their effectiveness and reliability for neuro-behaviorally impaired human populations.